1. Field of the Invention
This invention relates generally to an optical low-pass filter and, more particularly, to an optical low-pass filter suitable for discretely forming images which are picked up using an image pickup element such as an image pickup tube or plate in a video camera, electronic still camera or the like.
2. Description of the Prior Art
In general, in a video camera system or the like using a solid-state image pickup element having a discrete pixel structure, image data is spatially sampled to obtain an output image signal.
In this system, if the received image contains spatial components of frequencies higher than the sampling frequency, signals unrelated to the form and the colors of the object or, in particular, pseudo color signals are frequently generated. That is, frequency components (having frequencies higher than the Nyquist frequency) which cannot be sampled by the camera are not reproduced as image data and may cause waveform distortions (aliases), moire fringes, pseudo colors, and so on.
Conventionally, an optical low-pass filter is inserted in the picture taking optical system to suppress high frequency spatial components of the received image. A type of optical low-pass filter such as a crystal plate using double refraction is ordinarily used for this purpose.
FIG. 6 shows an optical system having an imaging system in which a conventional optical low-pass filter formed of a crystal plate utilizing double refraction is provided.
The optical system shown in FIG. 6 has an imaging lens 44, and an image pickup element 48 such as a CCD. The light receiving portion of the image pickup element 48 is covered with a checkerboard-like color filter 42. A protective cover glass 47 is disposed in front of the light receiving portion. An infrared light cutting filter 45 and an optical low-pass filter 46 (comprising a crystal plate and integrally attached to the infrared light cutting filter 45) are disposed between the imaging lens 44 and the image pickup element 48. As is well known, the low-pass effect of the optical low-pass filter 46 resides in separating ordinary and extraordinary rays from incident rays.
However, crystal plates are expensive and the use of a crystal plate increases the overall length of the optical system because it is necessary for the crystal plate to have a substantially large thickness to separate out a certain amount of incident rays.
Japanese Patent Laid-Open Patent Publication No. 53-119063 discloses an optical low-pass filter using a phase type diffraction grating. According to this publication, a desired low-pass effect can be obtained by forming a suitable diffraction image using a diffraction effect.
This type of diffraction grating optical low-pass filter offers a reduced manufacturing cost and a reduced thickness compared with crystal plate optical low-pass filters. Further, it is not necessary for the diffraction grating optical-low pass filter to be disposed at the rear of the imaging lens; it may be disposed at a freely selected position. It is therefore easy to limit the overall size of the imaging system using such an optical low-pass filter.
In video cameras or electronic still cameras, as described above, the infrared light cutting filter 45 (for luminosity compensation) and the optical low-pass filter 46 formed of, e.g., a crystal plate, are inserted in the imaging system independently of each other or in a combined state, as shown in FIG. 6. It is necessary for the infrared light cutting filter and the optical low-pass filter to have a certain substantial thicknesses to optimize their effects. Ordinarily, if a plurality of filters having certain thicknesses are provided in an imaging system, the mechanical and optical construction of the imaging system becomes complicated, resulting in difficulty in reducing the overall size of the imaging system.